Solid high-energy compositions, such as propellants, explosives, gasifiers or the like, comprise solid particulates, such as fuel particulates and oxidizer particulates, dispersed and immobilized throughout a binder matrix comprising an elastomeric polymer.
Conventional solid composite propellant binders utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents. As outlined in detail in U.S. Pat. No. 4,361,526, there are important disadvantages to using cross-linked elastomers as binders. Cross-linked elastomers must be cast within a short period of time after addition of the curative, which time period is known as the "pot life". Disposal of a cast cross-linked propellant composition is difficult except by burning which poses environmental problems. Furthermore, current state-of-the-art propellant formulations have serious problems that include, but are not limited to, use of nonenergetic binders, high end-of-mix viscosities, thermally labile urethane linkages, and extreme vulnerability to unscheduled detonation.
Cross-linked elastomers in which prepolymers are polyethers derived from oxetane derivatives and tetrahydrofuran (THF) are described in U.S. Pat. No. 4,483,978 issued to Manser. Urethane curing is achieved with isocyanates and additional cross-linking agents.
In view of inherent disadvantages of cross-linked elastomers polymers as binder materials, there has been considerable interest in developing thermoplastic elastomers suitable as binders for solid, high-energy compositions. However, many thermoplastic elastomers fail to meet various requirements for propellant formulations, particularly the requirement of being processible below about 120.degree. C., it being desirable that a thermoplastic polymer for use as a binder in a high-energy system have a melting temperature of between about 60.degree. C. and about 120.degree. C. Many thermoplastic elastomers exhibit high melt viscosities which preclude high solids loading and many show considerable creep and/or shrinkage after processing.
Thermoplastic elastomers (TPE's) typically obtain their thermoplastic properties from segments that form glassy domains which may contribute to physical properties adverse to their use as binders. Thermoplastic elastomers are block copolymers with the property of forming physical cross-links at predetermined temperatures. The classical TPE, e.g., Kraton, obtains this property by having the glass transition point of one component block above room temperature. At temperatures below 109.degree. C., the glassy blocks of Kraton form glassy domains and thus physically cross-link the amorphous segments. The strength of these elastomers depends on the degree of phase separation. Thus, it remains desirable to have controlled, but significant, immiscibility between the two types of blocks, which is a function of their chemical structure and molecular weight. On the other hand, as the blocks become more immiscible, the melt viscosity increases, thus having a deleterious effect on the processibility of the material.
Above-mentioned U.S. Pat. No. 4,361,526 proposes a thermoplastic elastomer binder which is a block copolymer of a diene and styrene, the styrene blocks providing a meltable crystal structure and the diene blocks imparting rubbery or elastomeric properties to the copolymer. This polymer requires processing with solvent, which is undesirable in that the propellant cannot be cast in a conventional manner, e.g., into a rocket motor casing. Furthermore, solvent-based processing presents problems with respect to removal and recovery of solvent.
There exists a need for novel thermoplastic elastomers which can be used as binders in high-energy compositions.